Lanthanum chromite, cell interconnects

Co-extruding a strip of lanthanum chromite based interconnect along the length of a YSZ microtube has also been demonstrated [45], although a number of difficulties remain. Firstly, the tubes are much weakened by the interconnect strip, and secondly the mixing of lanthanum chromite and YSZ at the boundary of the co-extruded materials leads to a dead-zone of material, about 350 pm in extent. Thus any microtubular cell design with co-extruded interconnect will require much further development to be successful. 

The interconnect material is in contact with both electrodes at elevated temperatures, so chemical compatibility with other fuel cell components is important. Although, direct reaction of lanthanum chromite based materials with other components is typically not a major problem [2], reaction between calcium-doped lanthanum chromite and YSZ has been observed [20-24], but can be minimized by application of an interlayer to prevent calcium migration [25], Strontium doping, rather than calcium doping, tends to improve the resistance to reaction [26], but reaction can occur with strontium doping, especially if SrCr04 forms on the interconnect [27],... 

Fergus JW. Lanthanum chromite based materials for solid oxide fuel cell interconnects. Solid State Ionics 2004 171 1-15. 

Cell Interconnect Pt Mn doped cobalt chromite Doped lanthanum chromite Plasma spray 10 X 10 cm/cm °C 100 pm thickness... 

Satisfactory conductivity is maintained up to 1800 °C in air but falls off at low oxygen pressures so that the upper temperature limit is reduced to 1400 °C when the pressure is reduced to 0.1 Pa. A further limitation arises from the volatility of Cr2C>3 which may contaminate the furnace charge. The combination of high melting point, high electronic conductivity and resistance to corrosion has led to the adoption of lanthanum chromite for the interconnect in high temperature solid oxide fuel cells (see Section 4.5.3). 

The lanthanum chromite interconnect strip is applied along the length of the tube by plasma-spraying. Since this is required to pass through the electrolyte and anode it is necessary to use appropriate masking during their deposition. The connection between cells is made via a soft nickel felt so that no dangerous mechanical stresses are placed on the tubes. 

Various alloys, cermets as well as lanthanum chromites with different dopants were used as interconnects in SOFC prototypes. The seals having different softening temperatures were elaborated and widely used for gas-tight assembling cells into stacks. 

The cell interconnect (doped lanthanum chromite) must be impervious to fuel and oxidant gases, and must possess good electronic conductivity. The interconnect is exposed to both the cathode and anode environments. Thus, it must be chemically stable under O2 partial pressures of about 1 to 10 atmospheres at 1,000 °C. The interconnect material is applied to the cathode tube as a narrow strip (see Figure 7-9, Figure 7-11) prior to depositing the electrolyte by masking the rest of the tube. Similarly, the interconnect strip is masked when the electrolyte is applied. 

The ceramic-type interconnect commonly used for planar SOFCs is based on lanthanum chromite. Recently, a LaxCai.xCryCoj.yOj solid solution has been developed. It is readily sintered in air at 1350 to 1500°C and appears to have superior stability and electrical properties compared to previous lanthanum chromite materials. Densities greater than 95% of theo-retieal are routinely achieved. The interconnect is machined into the desired shape to form the channels for gas flow. Figure 12.30 shows a sectional view of a 25-cell stack of 10 x 10 cm and its performance. A maximum output of 421 W is obtained. 

To satisfy these requirements, doped lanthanum chromite is used as the interconnection for cells intended for operation at about 1,000°C. Lanthanum chromite is a p-type conductor its conductivity is due to small polaron hopping from room... 

Kuo LJH, Vora SD, Singhal SC (1997) Plasma spraying of lanthanum chromite films for solid oxide fuel cell interconnection application. J Am Ceram Soc 80 589-593... 

Lanthanum chromite-based perovskite oxides (LaCrOs) have been widely recognized as promising interconnect materials for solid oxide fuel cells (SOFCs). The interconnects must separate fuel and oxidant gases and also have high electronic conductivity at high temperature (773-1273 K). Therefore, interconnects should meet the following requirements ... 

Indium oxide with different additives was proposed as a cathode material in 1956 [109] and frequently used (e.g. [110,107,108]). How ever, electronically conducting perovskites soon began to dominate the developments for both cathode and interconnect. The use of Lai- -Sr CoOs for the air electrode of solid oxide fuel cells marked the beginning [111], followed in 1967 by recommendations of PrCoOs [112] and of mixtures of the oxides of Pr. Cr, Ni and Co [113]. Strontium-doped lanthanum chromite, even now the most important ceramic interconnection material, was proposed by Meadowcroft in 1969 [114]. For cathodes, the situation in 1969 was summarised [115] as It Is apparent that a fully satisfactory air electrode for high temperature zirconia electrolyte fuel cells is still lacking. ... 

Lanthanum chromite has provided long lifetimes, as long as 69,000 h in Siemens Westinghouse tubular cells, at 900-1000°C. However, metallic interconnects have not yet shown equivalent lifetime performance. Improvements in metallic interconnect compositions and contact layers between cells/interconnects are still issues for materials development. In particular, the metal/ceramic interface in cells should have low corrosion, low contact resistance and low permeability of chromium species. Recent results have shown that optimised steels for SOFC applications are available and alkaline earth-free and cobalt-containing perovskites are the most suitable materials for contact layers however, their long-term performance under fuel cell operation conditions needs to be proven. 

Armstrong, T.R., Hardy, J.S., Simmer, S.P. and Stevenson, J.W. (1999), Optimizing lanthanum chromite interconnects for solid oxide fuel cells. Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells (SOFC-VI), Eds. S.C. Singhal and M. Dokiya. The Electrochemical Society Proceedings Series, Pennington, NJ, pp. 706-715. 

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